Replace point-to-point with collective mode in MPP_GATHER_PELIST_3D_

[abrooks1085]

Description
Using collective communication reduces manual indexing, simplifies memory management, and can improve performance. This PR updates the MPP_GATHER_PELIST_3D_ routine to leverage existing MPP_GATHER_1D(V)_ routines that already use collective communication.

Fixes # (issue)

How Has This Been Tested?
Please describe the tests that you ran to verify your changes. Please also note
any relevant details for your test configuration (e.g. compiler, OS). Include
enough information so someone can reproduce your tests.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• New check tests, if applicable, are included
• make distcheck passes